The present discussion of the prior art will make reference to the patent literature and technical papers listed at the end of this section.
The nanoscale graphene platelet (NGP) or graphene nano-sheet is an emerging class of nano materials. An NGP is a nanoscale platelet composed of one or more layers of a graphene plane, with a platelet thickness from less than 0.34 nm to 100 nm. In a graphene plane, carbon atoms occupy a 2-D hexagonal lattice in which carbon atoms are bonded together through strong in-plane covalent bonds. In the c-axis or thickness direction, several graphene planes may be weakly bonded together through van der Waals forces to form a multi-layer NGP. An NGP may be viewed as a flattened sheet of a carbon nano-tube (CNT), with a single-layer NGP corresponding to a single-wall CNT and a multi-layer NGP corresponding to a multi-wall CNT.
For more than six decades, scientists have presumed that a single-layer graphene sheet (one atom thick) could not exist in its free state based on the reasoning that its planar structure would be thermodynamically unstable. Somewhat surprisingly, several groups worldwide have recently succeeded in obtaining isolated graphene sheets [Refs. 1-9]. NGPs are predicted to have a range of unusual physical, chemical, and mechanical properties. Several unique properties associated with these 2-D crystals have been discovered. In addition to single graphene sheets, double-layer or multiple-layer graphene sheets also exhibit unique and useful behaviors. In the present context, single-layer and multiple-layer graphene sheet structures are collectively referred to as NGPs. Graphene platelets may be oxidized to various extents during their preparation, resulting in graphite oxide (GO) platelets. Hence, although NGPs preferably or primarily refer to those containing no or low oxygen content, they can include GO nano platelets of various oxygen contents.
Although practical electronic device applications for graphene are not envisioned to occur within the next 5-10 years, its application as a nano filler in a composite material is imminent. However, the availability of processable graphene sheets in large quantities is essential to the success in exploiting composite and other applications for graphene. The present patent application addresses issues related to the production of processable or dispersible NGPs.
The processes for producing NGPs and NGP nanocomposites have been recently reviewed by the applicants, Jang and Zhamu [Ref. 9]. Basically, there are four different approaches that have been followed to produce NGPs. Their advantages and shortcomings are briefly summarized as follows:
Approach 1: Formation and Reduction of Graphite Oxide (GO) Platelets
The first approach entails treating a laminar graphite material (e.g., in most cases, natural graphite powder) with an intercalant and an oxidant (e.g., concentrated sulfuric acid and nitric acid, respectively) to obtain a graphite intercalation compound (GIC) or, actually, graphite oxide (GO). The obtained GIC or GO is then subjected to exfoliation using either a thermal shock exposure or a solution-based graphene layer separation approach.
Technically, the acid-treated graphite is actually oxidized graphite or graphite oxide (GO), rather than pristine graphite. In the thermal shock exposure approach, the GIC or GO is exposed to a high temperature (typically 800-1,050° C.) for a short period of time (typically 15 to 60 seconds) to exfoliate the treated graphite. Typically, the exfoliated graphite oxide is then subjected to a further sheet or flake separation treatment using air milling, mechanical shearing, or ultrasonication in a liquid (e.g., water).
In the solution-based graphene separation approach, the GO powder is dispersed in water or aqueous alcohol solution, which is subjected to ultrasonication. Alternatively, the GO powder dispersed in water is subjected to some kind of ion exchange or purification procedure in such a manner that the repulsive forces between ions residing in the inter-planar spaces overcome the inter-graphene van der Waals forces, resulting in graphene layer separations.
In both the heat- or solution-induced exfoliation approaches, the resulting products are GO platelets that must undergo a further chemical reduction treatment to reduce (but normally not eliminate) the oxygen content. Typically even after reduction, the electrical conductivity of GO platelets remains much lower than that of pristine graphene. Furthermore, the reduction procedure often involves the utilization of undesirable chemicals, such as hydrazine. In some cases of solution-based exfoliation, the separated and dried GO platelets were re-dispersed in water and then cast into thin GO films. These films were exposed to a high temperature, high vacuum environment for de-oxygenation, but the resulting GO platelets were no longer dispersible in water or other solvents.
Examples of Approach 1 are briefly discussed below:                (a) Bunnell [10-12] developed a method in late 1988 that entailed intercalating graphite with a strong acid to obtain a GIC, thermally exfoliating the GIC to obtain discrete layers of graphite, and then subjecting the graphite layers to ultrasonic energy, mechanical shear forces, or freezing to separate the layers into discrete flakes. Although flakes as small as 10 nm were cited in the report [12], most of the flakes presented in the examples appeared to be thicker than 100 nm.        (b) In a similar manner, Zaleski, et al. [13] used air milling to further delaminate thermally exfoliated graphite flakes. The resulting structures exhibited a specific surface area of 35 m2/g, corresponding to an average flake thickness of approximately 25 nm.        (c) Horiuchi, Hirata, and co-workers [14-16] prepared nano-scaled graphite oxide (GO) platelets, which they coined as carbon nano-films. These films were prepared by a two-step process—oxidation of graphite and purification of the resulting graphite oxide. The oxidation of graphite was conducted using the now well-known Hummer's method [17], which entailed immersing natural graphite particles in a mixture of H2SO4, NaNO3, and KMnO4 to obtain GICs that actually were GOs. By hydrolyzing the GIC, functional groups, such as acidic hydroxyl groups and ether groups, were introduced into the inter-graphene layer spaces. Each of the graphite oxide layers became a multiple-charge anion, having a thickness of approximately 0.6 nm. When the excess small ions derived from the oxidants (e.g., NaNO3, and KMnO4) were thoroughly removed by a purification process, many layers tended to automatically separate from each other due to interlayer electrostatic repulsion. The resulting GO layers formed a stable dispersion in water. According to Horiuchi, et al. [14], single-layer graphene was detected.        (d) It may be noted that the approach of using electrostatic repulsion to separate graphene oxide layers was pursued earlier in 1998 by Liu and Gong [18], as a first step in their attempt to synthesize polyaniline-intercalated GO. In a 3-D graphite crystal, the inter-layer spacing (Ld) is 0.335 nm, which is known to increase to 0.6-1.1 nm if graphite is oxidized to produce GO. Further, GO is hydrophilic and can be readily dispersed in aqueous solution.        (e) Dekany et al. [19] observed that the inter-graphene spacing in GO was increased to Ld=1.23 nm when GO particles were dispersed in 0.05 N NaOH solution. When dispersed in a 0.01 N NaOH solution, the spacing was essentially infinite, likely implying that GO was completely exfoliated to become a disordered structure.        (f) Chen et al. [20] exposed GO to a temperature of 1,050° C. for 15 seconds to obtain exfoliated graphite, which was then subjected to ultrasonic irradiation in a mixture solution of water and alcohol.        (g) Jang et al. [21] thermally expanded GIC or graphite oxide to produce exfoliated graphite and subjected exfoliated graphite to mechanical shearing treatments, such as ball milling, to obtain partially oxidized NGPs.        (h) Thermal exfoliation as a way of producing nano-structured graphite was also attempted by Petrik [22].        (i) Thermal exfoliation of intercalated graphite or graphite oxide was conducted by Drzal et al. [23] using microwaves as a heat source. Microwave energy induced graphite exfoliation was disclosed earlier by Kwon, et al. [24].        (j) Aksay, McAllister, and co-workers [7-9, 45] also used thermal exfoliation of GO to obtain exfoliated graphite oxide platelets, which were found to contain a high proportion of single-layer graphene oxide sheets, based on the BET method with nitrogen gas adsorption in the dry state and in an ethanol suspension with methylene blue dye as a probe.        (k) Several polar organic compounds and polymers have been intercalated into inter-graphene or inter-flake spaces to form intercalated or exfoliated GO nanocomposites [e.g., 25]. Partial reduction of a polymer-GO to a polymer-graphene nanocomposite also could be accomplished electrochemically or chemically [18,26].        (l) Preparation of ultra-thin films by a layer-by-layer self-assembly approach from GO nano platelets and polymer electrolytes also has been investigated [27-31]. Although the original intent of these studies was primarily to fabricate self-assembled GO-poly (ethylene oxide) nanocomposites, their first step almost always involved exfoliation and separation of GO platelets. This was evidenced by the X-ray diffraction data of the resulting structures that showed complete disappearance of those diffraction peaks corresponding to graphite oxide or pristine graphite [27,29].        (m) Stankovich et al. [32] followed the approaches of Hirata et al. [16] to produce and disperse graphite oxide sheets in water to obtain stable colloidal dispersions. The graphite oxide dispersion was then reduced with hydrazine, a procedure previously used by Liu and Gong earlier [18], but in the presence of poly (sodium 4-styrenesulfonate). This process led to the formation of a stable aqueous dispersion of polymer-coated graphene platelets. Stankovich et al. [33] further developed a method to produce less hydrophilic GO platelets using an isocyanate treatment. However, unless stabilized by selected polymers, the chemically modified graphene sheets obtained through these methods tend to precipitate as irreversible agglomerates due to their hydrophobic nature. The resulting agglomerates became insoluble in water and organic solvents.        (n) Li et al. [34] overcame this issue by using ammonium to adjust the pH value of a dispersion of chemically modified graphene sheets in water, which served to maximize the charge density on the resulting graphene sheets. The resulting electrostatic forces acted to stabilize the aqueous suspension.        (o) Si and Samulski [35] reported a chemical route to aqueous solutions of isolated graphene sheets by reducing graphene oxide in three steps. (1) pre-reduction of graphene oxide with sodium borohydride at 80° C. for 1 h to remove the majority of the oxygen functionality; (2) sulfonation with the aryl diazonium salt of sulfanilic acid in an ice bath for 2 h; and (3) post-reduction with hydrazine (100° C. for 24 h) to remove any remaining oxygen functionality. The lightly sulfonated graphene can be readily dispersed in water at reasonable concentrations (2 mg/mL) in the pH range of 3-10. Isolated graphene sheets persist in the mixture of water and organic solvents including methanol, acetone, acetonitrile, thus making it possible to further modify its surface for applications such as reinforcements in composites. This is a very tedious process, nevertheless.        (p) Another very tedious process for the preparation of GO nano sheets, proposed by Becerril, et al. [46], entailed (1) intercalating-oxidizing graphite with a solution of NaNO3 and KMnO4 in concentrated H2SO4 for 5 days; (2) washing the oxidized graphite with 5 wt. % H2SO4 in water and reacting the washed oxidized graphite with a 30 wt. % aqueous solution of H2O2 to complete the oxidation; (3) removing inorganic anions and other impurities through 15 washing cycles that included centrifugation, discarding supernatant liquid, and re-suspending the solid in an aqueous mixture of 3 wt. % H2SO4and 0.5 wt. % H2O2 using stirring and ultrasonication; (4) carrying out another set of centrifugation and washing procedures three times using 3 wt % HCl in water as the dispersion medium and then one more time using purified water to re-suspend the solid; (5) passing this suspension through a weak basic ion-exchange resin to remove remaining acid; and (6) drying the suspension to obtain a powder.Approach 2: Direct Formation of Pristine Nano Graphene Platelets        (q) Without going through a chemical intercalation route, Mazurkiewicz [36] claimed to have produced graphite nano platelets having an average thickness in the range of 1-100 nm through high-pressure milling of natural flake graphite. However, no evidence was presented [36] to show that truly thin platelets (e.g., those <10 nm in thickness) were produced.        (r) Shioyama [37] prepared a potassium-intercalated GIC from highly oriented pyrolytic graphite (HOPG), initiated in situ polymerization of isoprene or styrene in the inter-graphene spaces, and then thermally decomposed inter-graphene polymer chains at a high temperature (500-1,000° C.). The volatile gas molecules served to exfoliate graphite layers, and, after the volatile gas escaped, isolated graphene sheets were obtained. Unfortunately, Shioyama did not discuss the thickness of the isolated graphene sheets.        (s) Jang, et al. [3,4] succeeded in isolating single-layer and multi-layer graphene structures from partially carbonized or graphitized polymeric carbons, which were obtained from a polymer or pitch precursor. Carbonization involves linking aromatic molecules or planar cyclic chains to form graphene domains or islands in an essentially amorphous carbon matrix. For instance, polymeric carbon fibers were obtained by carbonizing polyacrylonitrile (PAN) fibers to a desired extent that the fiber was composed of individual graphene sheets isolated or separated from each other by an amorphous carbon matrix. The resulting fibers were then subjected to a solvent extraction, or intercalation/exfoliation treatment. Graphene platelets were then extracted from these fibers using a ball milling procedure.        (t) Mack, Viculis, and co-workers [38,39] developed a low-temperature process that involved intercalating graphite with potassium melt and contacting the resulting K-intercalated graphite with alcohol, producing violently exfoliated graphite containing many ultra-thin NGPs. The process must be carefully conducted in a vacuum or an extremely dry glove box environment since pure alkali metals, such as potassium and sodium, are extremely sensitive to moisture and pose an explosion danger. It is questionable if this process is easily amenable to the mass production of nano-scaled platelets. One major advantage of this process is the notion that it produces non-oxidized graphene sheets since no acid/oxidizer intercalation or a high temperature is involved.        (u) In 2004, Novoselov, Geim, and co-workers [1,2] prepared single-sheet graphene by removing graphene from a graphite sample one sheet at a time using a “Scotch-tape” method. Although this method is not amenable to large-scale production of NGPs, their work did spur globally increasing interest in nano graphene materials, mostly motivated by the thoughts that graphene could be useful for developing novel electronic devices.        (v) Zhamu and Jang [54] developed a very effective way of exfoliating/separating NGPs from natural graphite and other laminar graphitic materials by exposing the material (without any intercalation or oxidation) to an ultrasonication treatment. This process may be considered as peeling off graphene layers at a rate of 20,000 layers per second (if the ultrasonic frequency is 20 kHz) or higher (if higher frequency). The resulting NGPs are pristine graphene without any intentionally added or bonded oxygen.Approach 3: Epitaxial Growth and Chemical Vapor Deposition of Nano Graphene Sheets on Inorganic Crystal Surfaces        (w) Small-scale production of ultra-thin graphene sheets on a substrate can be obtained by thermal decomposition-based epitaxial growth [40] and a laser desorption-ionization technique [41]. A scanning probe microscope was used by Roy et al. [42] and by Lu et al. [43] to manipulate graphene layers at the step edges of graphite and etched HOPG, respectively, with the goal of fabricating ultra-thin nano-structures. It was not clear if single graphene sheets were obtained using this technique by either group. Epitaxial films of graphite with only one or a few atomic layers are of technological and scientific significance due to their peculiar characteristics and great potential as a device substrate. The graphene sheets produced are meant to be used for future nano-electronic applications, rather than composite reinforcements.Approach 4: The Bottom-Up Approach (Synthesis of Graphene from Small Molecules)        (x) X. Yang, et al. [44] synthesized nano graphene sheets with lengths of up to 12 nm using a method that began with Suzuki-Miyaura coupling of 1,4-diiodo-2,3,5,6-tetraphenyl-benzene with 4-bromophenylboronic acid. The resulting hexaphenylbenzene derivative was further derivatized and ring-fused into small graphene sheets. This is a slow process that thus far has produced very small graphene sheets.        
There are several major issues associated with the aforementioned processes. These include, for instance:    (1) In most of these methods of graphite intercalation and exfoliation, undesirable chemicals are used. Consequently, a tedious washing step is required, which produces contaminated waste water that requires costly disposal steps.    (2) The GO nano platelets prepared by Approach 1 exhibit an electrical conductivity typically several orders of magnitude lower than the conductivity of pristine NGPs. Even after chemical reduction, the GO still exhibits a much lower conductivity than pristine NGPs. It appears that the preparation of intercalated graphite, which involves the use of an oxidizing agent such as nitric acid or potassium permanganate, typically and necessarily requires graphite to be heavily oxidized. Complete reduction of these highly oxidized graphite platelets to recover the perfect graphene structure hitherto has not been successfully attained.    (3) The NGPs produced by Approach 2 and Approach 3 are normally pristine graphene and highly conducting. However, other than the direct ultrasonication method developed by the applicants earlier [54], these processes are not amenable to large-scale production of NGPs with a reasonable cost.
In an attempt to address these issues, the applicants have previously disclosed a process for exfoliating a layered material to produce nano-scaled platelets having a thickness smaller than 100 nm [Ref. 57]. The process comprises: (a) charging a layered material to an intercalation chamber comprising a gaseous environment at a first temperature and a first pressure sufficient to cause gas species to penetrate into an interstitial space between layers of the layered material, forming a gas-intercalated layered material; and (b) operating a discharge means to rapidly eject the gas-intercalated layered material through a nozzle into an exfoliation zone at a second pressure and a second temperature, allowing gas species residing in the interstitial space to exfoliate the layered material for producing the platelets. One advantage of this process is the notion that it does not involve the utilization of un-desirable acids (such as sulfuric acid or nitric acid) or oxidizers (such as sodium chlorate or potassium permanganate). The gas environment used in the process can include a supercritical fluid, but the intercalated layered material must be rapidly discharged out of the intercalation chamber.
In a related topic, Kaschak, et al. [55] proposed a method of modifying graphite by introducing a supercritical fluid into interstices of chemically intercalated or intercalated/ oxidized graphite (rather than the original natural graphite). The interstices of intercalated and/or oxidized graphite had been expanded and chemically modified due to the presence of intercalant species (such as sulfuric acid) or oxidation-induced functional groups (such as carboxyl). Kaschak, et al. [55] did not teach about the approach of directly intercalating the un-treated natural flake graphite with a supercritical fluid; nor did they teach about the approach of intercalating and exfoliating graphite using the same supercritical fluid. The modified graphite as proposed by Kaschak, et al. [55] still required a high temperature exposure step, typically at 700-1,200° C., to exfoliate the intercalated and modified graphite.
Furthermore, Kaschak, et al. [55] did not provide any evidence to show the existence of nano-scaled graphite particles that they claimed they produced with this method. In particular, they claimed that “one advantage of the invention is that the aforementioned methods may be used to manufacture graphite in a form that has a thickness of less than about 10 microns, preferably less than about 1 micron, more preferably less than about 100 nm, even more preferably less than about 10 nm, and most preferably less than about 1 nm.” However, they did not fairly suggest the conditions under which graphite particles with a thickness less than 10 nm or 1 nm could be produced. This was truly a big claim and should have been supported by solid experimental evidence; unfortunately, absolutely no evidence whatsoever was presented.
Gulari, et al. [56] proposed a method of delaminating a graphite structure with a coating agent solubilized in a supercritical fluid. According to Gulari, et al. [56], the coating agent was a polymer, monomer, or oil. The method comprises diffusing a coating agent in a supercritical fluid between layered particles of a graphite structure and catastrophically depressurizing the supercritical fluid to delaminate the coating agent-intercalated graphite particles. However, Gultari, et al. [56] failed to mention anything about the thickness of the delaminated particles. It was not clear if and how graphite platelets with a thickness less than 100 nm could be produced with this method. Gulari, et al. presumed that a coating agent was needed to prevent the reformation of the covalent bonds between graphite particles when they were broken during delamination. This is rather confusing or misleading since it is well-known that the bonding between graphite layers is van der Waals force rather than covalent bond. Furthermore, a coating agent is problematic if a pure graphene product is desired. Gulari, et al. [56] did not teach if a supercritical fluid without a coating agent solubilized therein would be capable of delaminating graphite layers. Neither Kaschak, et al. [55] nor Gulari, et al. [56] discuss the properties of the resulting exfoliated graphite.
By contrast, after an intensive research and development effort, we have found that a supercritical fluid, alone without a coating agent, was capable of both intercalating and exfoliating a graphitic material without involving an additional intercalation or oxidation step (as required in Kaschak, et al. [55]). Further, this supercritical fluid-based process is capable of producing nano graphene platelets that are ultra-thin (<10 nm) and, in many cases, thinner than 1 nm. We have also discovered that it was not necessary to discharge the supercritical fluid-intercalated graphite out of a pressure chamber to achieve graphite exfoliation. Instead, exfoliation of graphite for the NGP production could be carried out by rapidly de-pressurizing or releasing the gas from the chamber where a graphitic material was tentatively intercalated by a supercritical fluid. By keeping the exfoliated graphite or NGPs in the chamber, the chamber can be re-pressurized and then de-pressurized to further separate multi-layer NGPs. This procedure can be repeated until all or most of the NGPs are single-layered structures.
Hence, it was an object of the present invention to provide a pristine nano graphene platelet material that has good electrical conductivity.
It was another object of the present invention to provide a process for mass-producing pristine NGPs without involving the use of any undesirable chemical.
It was a further object of the present invention to provide a process for producing ultra-thin NGPs (e.g., those with a thickness less than 1 nm).